In this kind of engine, for example, Patent Document 1 discloses a homogeneous charge compressed ignition engine, wherein a mixture gas in which, fuel and air are previously mixed is supplied to a combustion chamber in a cylinder to compress the mixture gas, thereby self-igniting the mixture gas.
The homogeneous charge compressed ignition engine has an advantage that thermal efficiency is high because operation can be performed at a compression ratio higher than that of a spark ignition type engine. Generation of NOx can also be restrained because a combustion temperature can be lowered. However, in the homogeneous charge compressed ignition engine, because the mixture gas is naturally ignited, it is extremely difficult to control ignition timing. The compression ignition can properly be performed only in an extremely narrow operating range because of a relationship between a torque (load) and an intake air temperature.
In the case of the homogeneous charge compressed ignition engine having a plurality of cylinders, cylinders differ from one another in a heat receiving property and a heat radiation property due to arrangement of each cylinder, whereby a difference in in-cylinder temperature is generated at compression ends of the cylinders. Generally, self-ignition timing is advanced when the in-cylinder temperature is higher at the compression end, and the self-ignition timing is delayed when the in-cylinder temperature is lower at the compression end. Therefore, a fluctuation in self-ignition timing is easily generated among the cylinders. The fluctuation in self-ignition among the cylinders lowers cycle efficiency and thermal efficiency and causes an increase in emission amount of an air-pollution substance, such as an unburned hydrocarbon, a carbon monoxide, and NOx, which is contained in an exhaust gas.
Patent Document 1: Japanese Patent Publication Laid-Open No. 2005-69097